Use 7z everywhere for consistency.
Use the correct stack of rootfs:s for the chroot browsers (Closes: #8152, #8152).

After installing incremental upgrades Tails' root filesystem consists
of a stack squashfs:s, not only filesystem.squashfs. When not stacking
them correct we may end up using the Tor Browser (Firefox) from an
older version of Tails, or with no Tor Browser at all, as in the
upgrade from Tails 1.1.2 to 1.2, when we migrated from Iceweasel to
the Tor Browser.

Based on patch contributed by sanic.
Optimize the startup of the chroot browsers.

Extracting and then compressing the whole langpack/omni.ja isn't
necessary and takes a lot of time. Tests show a 50% reduction in
startup time.
Refactor chroot browsers' cleanup().
Merge branch 'bugfix/8158-stacked-rootfs-vs-chroot-browsers' into bugfix/7951-refactor-chroot-browsers
Refactor chroot browsers' setup_chroot().
Refactor chroot browsers' set_chroot_browser_name().
Refactor chroot browsers' run_browser_in_chroot().
Separate some data from i2p-browser script.
Rename variable for consistency with i2p-browser script.

And use it consistently in all places instead of using the value as a
magic constant.
Rename more variables for consistency with i2p-browser script.
Refactor code for setting Torbutton's forced browser name.
Optimize startup of I2P browser a bit.

We don't have to fix the name in all branding files, only for the
locale we're actually gonna use.
Don't uninstall extensions for the chroot browsers.

We don't have to any more (after the TBB migration) since we
explicitly set up the ones we want to use. This should speed up the
start and reduce the memory footprint.
Don't set_chroot_browser_name() in i2p-browser script.

Apparently the name is dictated completely by Torbutton, so
set_chroot_torbutton_browser_name() is enough. This should speed up
the start and reduce the memory footprint.
Separate data from unsafe-browser script.
Separate more data from i2p-browser script.
Remove confusing line-break.
Refactor path into variable.
Comment.